<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
      /*当元素的所有子元素全部浮动时自动识别的高度为0,后面的元素会顶上来导致显示异常
         给元素添加overflow:hidden解决*/
     body>div{
       width: 200px;
        overflow: hidden;
       border: 1px solid red;
     }
     div>div{
       width: 50px;
       height: 50px;
     }
     #d1{
       background-color: red;
       float: left;
     }
     #d2{
       background-color: green;
       float: left;
     }
     #d3{
       background-color: blue;
       float: left;
     }
    p{
        background-color: #333333;
    }
    ul{
        list-style-type: none;
        padding: 0;
        overflow: hidden;
    }
    li{
        float: left;
        margin-right: 20px;
    }
    a{
        text-decoration: none;
    }
  </style>
</head>
<body>
<ul>
    <li><a href="">首页</a></li>
    <li><a href="">免费课</a></li>
    <li><a href="">直播课</a></li>
    <li><a href="">精品课</a></li>
    <li><a href="">线上班</a></li>
</ul>
<div>
  <div id="d1"></div>
  <div id="d2"></div>
  <div id="d3"></div>
</div>
<p>这是p标签</p>
</body>
</html>